TRH (thyrotropin releasing hormone) was discovered from the hypothalami as a peptide promoting secretion of thyroid stimulating hormone and prolactin of the pituitary glands A. V. Schally et al., Biochem. Biophys. Res. Commun., 25, 165-169 (1966)!. It has been reported that TRH is widely distributed in almost all sites of the brains other than the hypothalami, the spinal cords, the intestines, the pancreases, the adrenals and the like J. E. Morley et al., Life Sciences, 25, 1539-1550 (1979)!. The fact that TRH is also distributed in the brains and the spinal cords suggests that TRH plays an important role in the function of the central nerve system. For example, the actual administration of TRH to organisms has been found to bring about various central nerve actions in animals R. Guillemin, Recent Prog. Horm. Res., 33, 1-28 (1977)!. Also clinically, the administration of TRH has been reported to have therapeutic effect to schizophrenia K. Inagata et al., Arch. Gen. Psychatry, 35, 1011-1014 (1978)!, melancholia C. Hatanaka et al., Biochem. Biophys. Res. Commun., 60, 1350 (1974)!, spinocerebellar degeneration I. Sobue et al., Lancet, 1, 419 (1980)!, as well as the improvement of disturbance of consciousness. Further, the existence of TRH in the nerve synapsis in the brains suggests that TRH acts as a neurotransmitter T. Hokfelt et al., Eur. J. Pharmacol., 34, 389-392 (1975)!.
On the other hand, binding tests using isotope-labeled TRH reveal that TRH receptors widely exist not only in the pituitary glands, but also in the brains and the spinal cords N. A. Sharif, Ann. N. Y. Acad. Sci., 553, 147-175 (1989)!. Although there is a report that Scatchard plot analysis proves that TRH receptor molecules differ from one another in affinity between the pituitary and brain K. Funatsu et al., J. Neurochem., 45, 390-397 (1985)!, it is also reported that only one kind of receptor exist in the pituitary and brain N. Ogawa et al., Peptides, 5, 743-746 (1984)!. TRH receptor-cDNAs have been cloned from pituitary tumor cell line of mice R. E. Straub et al., Proc. Nat. Acad. Sci.. U.S.A., 87, 954-958 (1990)! and rats P. Pena et al., Biochem. J., 284, 891-899 (1992); and D. Zhao et al., Endocrinology, 130, 3529-3536 (1992)!. Mouse TRH receptor cDNA has been reported to code for 393 amino acids. On the other hand, two isoforms are coded by the rat TRH receptor cDNA, i.e. 412 amino acids and 387 amino acids respectively, which are considered to be produced by alternative splicing P. Pena et al., J. Biol. Chem., 267, 25708-25708 (1992)!. Further, primary sequences of amino acids encoded by these cDNAs suggest that TRH receptor is a member of G-protein couple receptors with 7 transmembrane domains. It is not known whether receptor subtypes exist in addition to the TRH receptors. Certain experiments using tumor cells GH.sub.3 of the rat pituitary glands have been reported to provide an indication that a signal transduction of the TRH receptors is conjugated with an inositol phosphate turnover and calcium mobilization T. F. J. Martin et al., J. Biol. Chem., 261, 10141-10146 (1986)!. It is reported that the TRH receptors in the rat brain are conjugated with the inositol lipid metabolic turnover and the formation of cAMP M. Mori et al., Research Communications in Chemical Pathology and Pharmacology, 71, 17-26 (1991)!.
Synthesis of certain derivatives of TRH have been reported, and binding activity to TRH receptors has been examined. It has been observed that some of these compounds bind to the TRH receptors S. M. Simasko and A. Horita, Life Sci., 30, 1793-1799 (1982)! and also have effect on the central nerve system including acetylcholine-release stronger than TRH M. Shikata et al., Japan. J. Pharmacol., 32, 883-891 (1982); and P. H. Huston et al., Neurosci. Lett., 116, 149-155 (1990)!. Some agonists or antagonists to the TRH receptors may be useful as drugs acting on the human central nerve system. When the agonists or the antagonists to the TRH receptors as the drugs acting on the human central nerve system are screened, it is conceivable to use human tissue or cells in which the TRH receptors are expressed, thereby screening substances binding to the receptors as primary screening. However, it is generally not possible to obtain such human tissue. Also, culture cells in which the human TRH receptors are expressed are little known. On the other hand, it is possible to use animal tissue or cells in which the TRH receptors are expressed, thereby conducting screening. In this case, however, the difference in characteristics of the receptors between animal species, so-called species specificity of the receptors, is a problem. In addition to the above, no culture cells in which the human TRH receptors are expressed have been available, so that there has been no means for examining the signal transduction of the human TRH receptors at all.
As a means for solving such a problem, if human TRH receptor genes can be cloned, the human TRH receptors can be expressed in animal cells and the like using them by an appropriate means. It is therefore considered that screening and studies of the human TRH receptor agonists and antagonists can be largely advanced. However, there is no information for human genes of the receptor proteins capable of binding human TRH as yet.